In mesa-type arrays of Group II-VI alloy semiconductor photodetectors a multilayered structure is first patterned and then etched to form a plurality of optically differentiated radiation photodetector sites or pixels. For a photovoltaic type of detector each optically differentiated pixel is comprised of portions of at least two layers of Group II-VI material that are contained within at least one mesa structure. The mesa has sloping sidewalls and exemplary dimensions for a base portion of 60 micrometers square and a height of five micrometers. At least two layers have different types of electrical conductivities (e.g., one layer is p-type and the other is n-type). The resulting p-n or n-p, heterojunction is reverse biased during use for collecting and reading out charge carriers (electrons or holes) that result from the absorption of IR radiation within at least one of the layers. For a backside illuminated array, the IR radiation is made to be incident upon a surface of a transparent substrate that is opposite to a surface that supports the mesa structures. The IR radiation passes through the substrate and into the radiation absorbing layer within a given mesa.
As the volume of the radiation absorbing layer within each mesa is made larger the collection efficiency of the photodetector can be increased, as there is a greater probability that a photon will be collected and absorbed. However, an increase in the volume of the radiation absorbing layer also results in a tendency towards increased thermal noise generation and other undesirable effects. More particularly, both the signal (photocurrent) and the leakage or dark current are proportional to pixel area. However, the noise is equal to the square root of the dark current. Therefore, the signal/noise ratio increases as the square root of the area as the area increases.
It can thus be appreciated that it is desirable to satisfy two conflicting goals, i.e., to maximize the collection efficiency of each photodetector, while at the same time minimizing a volume of the radiation absorbing layer.
It is known to employ a microlens element with each photodetector pixel to focus the incident radiation into a smaller collection volume. However, the fabrication of such microlenses is difficult, and often may not present a practical approach to improving the detector performance.
U.S. Pat. No. 5,721,429, issued Feb. 24, 1998, entitled "Self-Focusing Detector Pixel Structure Having Improved Sensitivity", by William Radford and Jerry A. Wilson teaches an array of mesa-type photodetectors that are constructed to provide for an internal reflection of incident electromagnetic radiation into a collection volume of the photodetector, thereby increasing the collection efficiency of each photodetector, while at the same time enabling a reduction in the volume of the radiation absorbing region of each photodetector.